(1) Field of the Invention
This invention relates to an image sensor and an image pickup apparatus such as an in-situ storage image sensor for performing high-speed photography, for example. More particularly, the invention relates to a technique for obtaining images of high signal-to-noise ratio in time of high-speed photography.
(2) Description of the Related Art
Conventional image pickup apparatus include a video camera (ultra high-speed video camera) which realizes a high-speed photography of 1 million frames per second, for example. This image pickup apparatus realizes high-speed photography with an image sensor called “in-situ storage image sensor” to be described hereinafter. The in-situ storage image sensor has a plurality of unit pixels including photosensitive pixels for performing a photoelectric conversion of incident light, and storage pixels for storing, on a frame-by-frame basis, signal electrons generated in the photosensitive pixels for a plurality of frames (images) photographed at different times. This image sensor outputs the signal electrons from the respective storage pixels after high-speed photography. Thus, after storing and recording a plurality of frames as classified in the image sensor, the plurality of recorded frames are read from the image sensor. In this way, a continuous image recording may be carried out at high speed without limitations imposed by read time.
Exposure time is shorter in high-speed photography (e.g. 1 million frames per second) than in ordinary photography. Thus, it often is the case that a sufficient quantity of light cannot be secured for high-speed photography because of the performance of an illuminating device used in the high-speed photography, and the need to avoid heating of a photographic subject due to illumination light.
Phenomena taken as objects of high-speed photography often are repetitive phenomena. Such repetitive phenomena are varied and include, for example, a reciprocating motion of an engine piston and a rotary motion of a rotor in a motor. In observing a repetitive phenomenon, it is well known to improve a signal-to-noise ratio by integrating signals generated at points of time when a motion is in the same phase, i.e. signals (highly relevant signals) generated at points of time when the phenomenon is in the same state. It should be possible to observe images with a high signal-to-noise ratio, even where an illuminating device of low illuminance is used, by applying this principle to high-speed photography and integrating signals of each corresponding phase. FIG. 1 is an explanatory view of an integrating process for a repetitive phenomenon carried out by a conventional image pickup apparatus using the above principle. For expediency of description, FIG. 1 shows only one photosensitive pixel 101, and does not show a plurality of storage pixels provided for this photosensitive pixel 101. As shown in FIG. 1, the conventional image pickup apparatus stores, on a frame-by-frame basis in the plurality of storage pixels, signal electrons generated in the photosensitive pixel 101 for a plurality of frames (images) photographed at different times by high-speed photography. An output amplifier 103 amplifies and outputs the signals (signal electrons) from the storage pixels successively and individually. An analog-to-digital converter 105 converts the analog output electric voltage (or current) signals from the output amplifier 103 to digital signals. A signal integrator 107 integrates digital signals of a corresponding phase i.e. digital signals generated at different points of time when a phenomenon is in the same state, among the digital signals from the analog-to-digital converter 105, and outputs the digital signals as a final output. The conventional image pickup apparatus has no other way but to integrate image signals of each corresponding phase read from the image sensor as noted above.
However, the conventional apparatus constructed as described above has the following drawback.
In the conventional image pickup apparatus, as shown in FIG. 1, signals (signal electrons) of each corresponding phase acquired through a series of observations are read from the image sensor a plurality of times (corresponding to the number of integrating operations). Each time the signals are read, read noise (1/f noise, shot noise, etc.) is superimposed on the signals at the output amplifier 103. This constitutes a problem of lowering the signal-to-noise ratio.